This invention relates to improving electrical signal transmission for systems having electrical connectors with parallel adjacent contacts. More particularly it relates to the partial cancellation of pair to pair interference or cross talk induced into a signal carrying cable pair by FCC type modular jacks and plugs, or other input/output signal connectors while maintaining proper longitudinal balance and characteristic impedance within the connector system.
The Federal Communications Commission has adopted certain architectural standards with respect to electrical connectors utilized in the telecommunications industry so as to provide intermatability. The connectors most commonly utilized are FCC type modular plugs and jacks. The plug is normally terminated to a cable having a plurality of parallel conductors which may be connected to a telephone handset or other communications device. The conductors are paired and each pair forms a signal loop. Each pair in the cable normally consists of two adjacent twisted conductors. This arrangement in the cable results in certain electrical characteristics of the cable including its characteristic impedance and longitudinal balance. The corresponding jack is commonly mounted to a panel or a printed circuit board which in turn is connected to a telecommunications network. A typical FCC jack is described in U.S. Pat. No. 4,648,478.
Often a plurality of jacks, such as six, are placed adjacent to one another in a housing with the printed circuit board forming the backside of the housing. Each jack includes a plurality of elongated contacts which are closely spaced and parallel to one another. A typical jack having eight adjacent and parallel contacts is shown in FIG. 1. Jack 10 includes electrical contacts 12 which are housed in the hollow portion of the jack and include at least two pairs of parallel contacts. Each pair 14 and 20 form separate communication circuits, each of which will be referred to herein sometimes as a signal pair. Pair 14 includes adjacent conductors 16 and 18 and pair 20 includes conductors 22 and 24 which flank pair 14. Each of these contacts extend through the backside 26 of jack 10 and are soldered to circuit board 28, as shown in FIG. 2. The lengths of the contacts running through the jack are predetermined by connector design considerations.
When an electrical signal of a given frequency is applied to a pair of conductors, an unequal portion of signal energy is transmitted to the individual conductors of an adjacent pair by each conductor of the signal pair. This transmission is primarily due to the capacitive and inductive couplings between adjacent conductors being substantially higher than the couplings of the other conductor of the signal pair resulting in a phenomenon known as cross talk. This phenomena is further exacerbated when both conductors of the signal pair are placed adjacent to and outside of, or flanking, opposing conductors of the other signal pair. For ease of discussion adjacent conductors which are not a signal pair in themselves are referred to herein as a cross-talking pair. For example conductors 16 and 22 form a cross-talking pair and conductors 22 and 24 form a signal pair. The extent of the cross talk is governed by such parameters as the space between the adjacent conductors, the dielectric constant of the material between such conductors and the distance in which such conductors are closely spaced and parallel to one another. Furthermore, the extent of the cross talk is a function of the frequency of the signal on either or both of the signal pairs. Cross talk increases logarithmically as the frequency of the signal increases and is commonly expressed as 10 .times. log of the ratio of the cross talk energy divided by the signal energy (decibels or dB).
As FCC modular plugs and jacks are utilized more and more in high frequency data and communication applications, cross talk which arises in the adjacent and parallel contacts of the jack, and to a lesser degree in the plug, has become a problem in the industry. Furthermore, because of miniaturization as well as FCC rules, these jacks are quite small resulting in a very close spacing of the contacts which worsens the problem.
A current industry standard permits no more than -56 dB of near end cross talk at 16 Megahertz (MHz). Using good prior art design techniques, such as that shown in FIG. 3, one is able to reach -50 dB of cross talk at 16 MHz utilizing a commonly available modular plug and jack combination. Many designers believe that the routings of the individual circuit paths forming a single signal pair on the circuit board should generally be parallel to each other and the signal pairs should be separated from other pairs as much as possible and be non-parallel to them. That type of design technique on the circuit board does nothing to reduce the cross talk which arises within the connector which is attached to the circuit board but simply, at best, maintains the status quo.
It has been found that cross talk coupling induced by the modular plug and jack interface can be reduced to a great extent by judicious placement of conductors after they exit the modular jack so as to induce signals of opposite phase to those which were induced inside the plug and jack. In the case where the exit from the modular jack is to a printed circuit board, this conductor placement can be effected by using routing methods so as to insure consistent performance in the mass produced part. To decrease cross talk, the circuit traces that form both pairs should be routed in a pattern that is opposite in polarity to the pattern that produces cross talk in the jack and plug. Thus, a positive polarity signal that is induced in the jack is reduced by a negative polarity signal induced by traces on the printed circuit board. It has been demonstrated that improvement in cross talk may be obtained by utilizing this technique to cancel cross talk induced between pairs 1 and 2 of a four pair plug and jack interface wired according to the E.I.A.- A Standard. This wiring standard requires pair 2 to straddle pair 1 causing poor cross talk. The routing pattern of the circuit board places the traces of pair 2 on opposite sides of the corresponding conductors of pair 1 as to which they were positioned inside a plug and jack, thus increasing opposing polarity coupling and thereby reducing the net induced cross talk. The above-described technique of cross talk reduction is described and claimed in U.S. patent application Ser. No. 07/855,893, filed Mar. 23, 1992 by Kenneth W. Brownell and Sterling A. Vaden, titled Low Cross Talk Electrical Connector System, which application is assigned to Superior Modular Products, Inc., assignee of the present application. U.S. application Ser. No. 07/855,893 is incorporated herein by reference.
It has been found that by utilizing the cross talk reduction techniques as set forth above, while having a desirable effect on the reduction of cross talk, the rerouting on the circuit board contributes to an impedance mismatch of the entire transmission system, including the cable, connector, and printed wiring board, thereby negatively affecting structural return loss, voltage standing wave ratio, and combined system attenuation. Table 1, set forth below, shows near end cross talk (NEXT) performance and structural return loss (RETURN LOSS) for two similar sample board assemblies. Assembly 100067 REV P1 utilizes strictly prior art paired traces on the circuit board 30, as shown in FIG. 3, while Assembly 100067 REV B utilizes split pairs, as shown in FIG. 5, which is the subject matter of U.S. patent application Ser. No. 07/855,893.
TABLE 1 ______________________________________ TABLE 1A PAIRS NEXT 100067 REV P1 NEXT 100067 REV B ______________________________________ 1-2 -55.5 dB -65.2 dB 1-3 -59.1 -57.3 1-4 -63.2 -62.8 2-3 -74.6 -78.5 2-4 -63.8 -60.1 3-4 -62.3 -67.3 ______________________________________ TABLE 1B RETURN LOSS 100067 REV P1 RETURN LOSS 100145 REV B ______________________________________ PAIR 1 -38.1 dB PAIR 1 -37.1 dB PAIR 2 -34.8 PAIR 2 -28.0 PAIR 3 -38.0 PAIR 3 -36.7 PAIR 4 -35.8 PAIR 4 -36.1 ______________________________________
Table 1 refers to next pairs 1-2 (Table 1A) and return loss for pair 2 (Table 1B). It is clear that the return loss is substantially improved for pair 2 in the REV P1 sample where parallel runs are utilized when compared to a split pair. This parallel routing, however, limits the amount of cross talk reduction that can be effective in a given board length or area because only one conductor of each pair is in close proximity with the other cross-talking conductors. Thus, pair 1-2 cross talk improvement is further enhanced when the pairs are split, however, return loss is worsened in pair 2. In the embodiments set forth above, pair 1 is the innermost pair, pair 2 flanks pair 1, and pairs 3 and 4 are the two outermost pairs. A schematic of a prior art paired routing of 100067 REV P1 is shown in FIG. 3 with pair 1 indicated as 31 and pair 2 indicated as 33. A schematic of 100067 REV B showing a split paired routing as taught in U.S. application Ser. No. 07/855,893 is shown in FIG. 6 with pair 1 being represented as 35 and pair 2 represented as 37.
It is therefore desirable to reduce cross talk without sacrificing return loss.